You have two options :
1. If you want to version your actionhandler implementations and possible helper classes along with your process definition, you should put the source tree in the *.par archive.
2. If you don't want to do this, put the *.class files simply in the classes WEB-INF/classes folder or in a *.jar file in the WEB-INF/lib folder.
or if you want them accessible from other processes as well, you can even put them on the classpath afaik